Whether we should have grammar schools or not is a complex subject, with lots of competing arguments.
I think they do help bright children from ordinary backgrounds access a good education that their parents couldn't otherwise afford. My friendship group my grammar were predominantly from working-class backgrounds (not free school meals, but not professional/upper MC backgrounds either).
If parents are motivated and able, they can tutor them through. I'm not sure why there is such an outcry about paid tutoring, when you can just buy the workbooks, take the answers out, give them to your child to do, and then go through the answers together. I couldn't afford tutoring as a single parent albeit one working full-time and did this, and my child qualified for grammar.
As someone mentioned in a comment above, there is always some type of selection. If not by ability through a grammar system, then by money through more expensive housing in MC catchment areas, or through jumping through religious hoops.
However, the secondary modern/upper schools do not get the "parity of esteem" that was once hoped for. Most staff and many students work exceptionally hard in these institutions, and face greater barriers. Grammar schools find it easier to recruit due to the high status afforded to them by the excellent grades they achieve through creaming off the academically most able.
Pupils can feel like they are failures before they've started year 7 and can internalize this as their "worth". However - most state primaries send very few children to grammar schools, so maybe this impact is limited.
I also see more mental health issues at girls' GSs, more anxiety over how they present to others and their identities, although this is also anecdotal.